Omega Engineering CY670 Series Installation And Operation

CY7/670 Series Temperature Sensors
Application Notes
M-4447/0307
INSTALLATION AND OPERATION
Three aspects of using a temperature sensor are
critical to its optimum performance:
• the proper electrical and thermal installation of
the connecting leads that run to the sensor
• the actual mounting of the sensor to the sample
assembly
• the measurement electronics used for reading
and recording temperature data from the sensor
Connecting Leads
Although the majority of the CY7/CY670 series
sensors are two-lead devices, measurements are
preferably made using a four-wire configuration to
avoid all uncertainties associated with lead
resistance. This is done by using four connecting
leads to the device and connecting the V+ and I+
leads to the anode and the V– and I– leads to the
cathode as shown in Figure 1. The exact point at
which the connecting leads are soldered to the
device leads results in negligible temperature
measurement uncertainties.
In a two-wire measurement configuration, the
voltage connections (point A in Figure 1) are made
near or at the current source, so only two leads
are actually connected to the device. Some loss in
accuracy can be expected since the voltage
measured at the voltmeter is the sum of the diode
voltage and the voltage drop across the
connecting leads. The exact temperature
uncertainty will depend on the temperature range
and lead resistance. For a 10-ohm lead
resistance, the diode voltage will be offset by 0.1
mV, which gives a negligible temperature error at
liquid helium temperature but a 50 mK error near
liquid nitrogen temperature. Note the PI and CY
adapter can be used only in a two-wire
configuration.
Figure 1. Four-Wire Configuration for CY7/670 Series Sensor Installation
An excessive heat flow through the connecting
leads to any temperature sensor can create a
situation where the active sensing element (for the
CY7/670 series this is the diode chip) is at a
different temperature than the sample to which the
sensor is mounted. This is then reflected as a real
temperature offset between what is measured and
the true sample temperature. Such temperature
errors can be eliminated by proper selection and
installation of the connecting leads.
In order to minimize any heat flow through the
leads, the leads should be of small diameter and
low thermal conductivity. Phosphor-bronze or
manganin wire is commonly used in sizes 32 or 36
AWG. These wires have a fairly poor thermal
conductivity yet the resistivities are not so large as
to create any problems in four-wire
measurements.
Lead wires should also be thermally anchored at
several temperatures between room temperature
and cryogenic temperatures to guarantee that heat
is not being conducted through the leads to the
sensor. A final thermal anchor at the sample itself
is a good practice to assure thermal equilibrium
between the sample and the temperature sensor.
Note that the CU, CY, SO, and DI mounting
adapters serve as their own sample thermal
anchor.
I the connecting leads have only a thin insulation
such as vinyl acetal or other varnish type coating,
a simple thermal anchor can be made by winding
the wires around a copper post or other thermal
mass and bonding them in place with a thin layer
of CYAV varnish. There are a variety of other
ways in which thermal anchors can be fabricated;
a number of guidelines can be found in detail in
the following references.